Trying to send a meeting request from my calendar to a co-worker, using Exchange Online Create Calendar Item for Office 365. I set-up my user with impersonation. Still I get an error "The username or password is invalid." I'm using the upn email address to authenticate.

In my case, the user account I'm using to authenticate with Exchange Online has permission to impersonate every user, but I still get this cryptic error message:

Error when creating calendar item: Reason: The service is not able to generate a response for the request. Error:Unable to create calendar item. Error: An internal server error occurred. The operation failed., Mailbox '.....'